A straw is constituted by a long narrow tube, used in particular in the field of artificial insemination, to receive a determined quantity of animal semen.
When supplied to users (insemination centers) for filling, these tubes generally have one of their ends closed. Depending on the application, the tube is filled with animal semen or with some other biological liquid such as viruses or an embryo in a liquid medium. After the other end of the straw has been closed by ultrasound welding or by any other technique known to the person skilled in the art, it is frozen for later use.
After being manufactured, straws are sterilized, generally by using gamma rays or by ionization, and they are packaged in sachets.
The straws most commonly in use have one end closed by a plug engaged inside the tube. The plug is generally constituted by a powder suitable for being transformed into a paste that is impermeable and leakproof. An example of material used is polyvinyl alcohol.
The powder is interposed between two wads of fiber material such as cotton. Such a material is selected to be permeable to liquids and to air. The wads are said to be external and internal, respectively.
For a straw of common type having a total length of 133 mm, the plug generally occupies a length of about 14 mm to 15 mm, with the remainder of the straw, referred to as the "working" volume, being intended to receive the biological liquid.
The method of filling such straws generally consists in applying suction to the end which is closed by the plug. To do this, said end is connected to a source of suction. A hollow needle connected to a source for feeding biological liquid, such as animal semen, is inserted into the other end.
In this way, the suction established through the permeable plug sucks the liquid into the straw. The liquid moistens the internal wad of the plug and also moistens the powder. The powder becomes progressively solid and impermeable.
Once the straw has been filled, it is closed. It can then be stored at very low temperature.
Nevertheless, implementing the above technique suffers from a drawback. After filling, and during unfreezing of the straw, a certain amount of expansion takes place, such that the external wad tends to move out of the tube. The straw must then be discarded since it cannot be used for insemination. It can even happen that on absorbing the liquid, the volume of the powder doubles, pushing away both the external wad and the internal wad, thereby reducing the working volume available for the liquid by a corresponding amount.
To mitigate that drawback, proposals have been made to interrupt the process of filling the straw only once a major portion of the external wad has soaked up the liquid. The wad then swells and deforms locally, thus opposing any tendency to move out of the tube.
Nevertheless, in spite of the precautions taken it sometimes happens that the wad does come out.
Further, by acting in that way, about 7% of the total volume of liquid inserted into the straw is retained in the plug and is therefore not usable, in particular for insemination purposes.